Imprintable objects can be made from various materials, for example plastic, ceramic, glass, wood, metal.
The imprinting of plastic parts, or of their surfaces, can be accomplished by the application of a variety of printing technologies. Known methods are the conventional printing methods such as, for example, the offset, pad, or silk screen methods.
Printing machines, in particular inkjet printers, are known for the printing of individual print patterns, especially for smaller part volumes. Inkjet printers are characterized by the selective application of liquid, in particular a jet of ink, onto a recording material, the inkjet printer being a matrix printer in which, by the targeted ejection or deflection of small ink droplets, a print pattern is generated on a surface; conventional inkjet printers are not suitable for imprinting plastic parts.
The plastic parts are, for example, electronics housings or housing covers for interface technologies, and labels, so-called self-adhesive or clip-in identification strips, identification cards, cable ties, slide-in signs, signage panels, terminal strip identifiers, designation sleeves, or other shaped parts for the identification or marking of, for example, modular terminal blocks, printed terminal blocks of all types, and converters, to name only a few.
The labels or signs are detachable secured, depending on their shape, on a variety of inscription carriers, labels or signs of film-like configuration being detachably adhered onto ribbon-shaped inscription carriers. Plastic signs or labels in particularly configured shapes are arranged in retaining apparatuses, in particular in frame-like inscription carriers, and attached thereto, for example, with defined break points. The retaining apparatuses thus contain shaped parts that, because of their conformation, are unsuitable or too small to be separately inscribed in a printer. They are therefore joined to a carrier element, the carrier element having a format that can be conveyed by the printer or the transport device.
The existing art describes the imprinting of objects, in particular of plastic parts, by silk screen and pad printing. EP 0 991 063 B1, in which the imprinting of optical data media with UV-curable ink is described, may be recited here as a representative of many examples. The disadvantage is that these printing methods are suitable only for the imprinting of print patterns that are always the same, and not for the objects previously mentioned. The aforesaid printing methods all operating with special printing tools that are not only complex and expensive, but also inflexible as regards adaptation of a print pattern to different predefined print patterns.
Pen plotters are also known from the existing art. A pen plotter is designed for print patterns on paper, generally DIN A3 to A0. In special cases, detachable labels adhesively bonded onto flat inscription carriers can also be imprinted. The pen plotter uses for this purpose an ink pen that is mounted on a carriage. The carriage slides along a bar that either can be displaced over the entire width of the paper, or is installed fixedly. The disadvantage of pen plotters is that they are slow when imprinting objects, and cumbersome to handle. The solvent-containing ink that is used additionally results in disruptions in the inscribing process, for example because pens have dried out. Pen plotters are therefore seldom used in the commercial sector.
GB 2235 163 A describes a plotter for imprinting plastic cassettes, in which plotter imprinting is accomplished by way of a thermal method with a heatable plotter pen and a carbon ribbon. The disadvantage here is that this method can be applied only to plastic, and furthermore that only a low resolution and printing speed can be implemented. The low printing speed results from the fact that the plotter pen must travel to each letter individually, in the manner of a plotter.
Printing machines that adapt commercially available inkjet printers are also known. These inkjet printers utilize water-based ink. With this printing method, the inscription carrier is heated, after the printing operation is complete, so greatly that the aqueous component of the ink is evaporated. EP 0 619 849 B1 describes an inkjet recording device that is equipped with a combined heating apparatus and blower unit for drying the printed-on ink with hot air. A disadvantage is that the inscription carriers must be heated to a temperature above 100 degrees Celsius in order for the water component contained in the ink to evaporate. The inscription materials are very highly stressed by the heat, and can distort or change shape. The adhesion of aqueous inks onto plastic surfaces is also insufficient. In order to protect the objects from excessive heating during imprinting, DE 43 42 643 C2 describes a method that fixes the printed-on ink with a low-heating radiation, the ink being fixed by a photochemical reaction. Fixing is accomplished by the use of a UV radiation source, eliminating the print-delaying waiting phase with thermal fixing. A UV radiation source downstream from the printing process is used for this purpose. There is a physically close association between the ink application unit and the UV radiation source: this has a disadvantageous effect on the ink application unit because of the heat generated by the UV radiation source. The heat that is generated acts unimpededly in all directions, and it is not possible to imprint various objects with this inkjet printer.
To prevent unimpeded propagation of the heat that results from radiation sources, DE 200 22 158 U1 describes a drying unit that is usable with an infrared radiation source. This involves a separate drying unit, equipped with a passive cooling agent, that is placed downstream from inkjet printers. The disadvantage of this drying unit is that it is usable only as a standalone unit for sheet paper drying, at a distance of approximately 20 to 30 cm from the inkjet printer. The drying of other imprinted objects is not possible.
In a further embodiment, DE 198 23 195 C2 describes a method and an apparatus for imprinting plastic workpiece surfaces. The method and apparatus are specifically designed only for the imprinting of profiled strips, the latter's surface being pretreated with a plasma process for better adhesion of the ink. Treatment of the surface with UV radiation for faster drying and curing of the ink does not occur. The inkjet printer is also not suitable for imprinting the objects mentioned above.
DE 101 15 065 describes a method and an apparatus for imprinting cassettes or specimen slides for histological preparations and/or glass specimen slides for microscopic thin sections, in which a computer device is provided for controlling the printing device, and the printing device comprises an inkjet printer for imprinting the cassettes and/or specimen slides. The ink is predried via a hot-air drying system, and completely dried via a flash device. The processing speed in the printing device is, however, greatly limited by the hot-air drying system. The hot-air and flash device is not integrated into inkjet printers. The inkjet printer used in the printing device is moreover a conventional inkjet printer that is equipped with a stationary ink tank on the printer frame to supply the print head. Examples of stationary ink tanks are described in the documents DE 199 23 291 B4 and DE 199 16 219 C2. Stationary ink tanks have the disadvantage that they are equipped with complex individual technical parts in order to generate a negative pressure in the print head or at the nozzle device. Such embodiments of ink tanks are too complicated in terms of their design and construction, and thus too expensive to manufacture.
A further problem with achieving a high-contrast printed image is that of ensuring a negative pressure of a few millibars that is present at the nozzle device of the printing apparatus. As known from the existing art, the negative pressure can be ensured by an ink tank equipped with an ink absorber. An ink tank of this kind is described in U.S. Pat. No. 4,771,295, such embodiments of ink tanks being arranged physically above the nozzle device in the printing machine. The absorber used in the ink tanks, which is manufactured in a complex process, possesses an essential disadvantage. The absorber is made up of a foam block that has only limited resistance with respect to solvent-containing ink. Because the objects to be imprinted are made of various materials, however, only solvent-containing inks are suitable for the printing method.
In order to mark plastic materials it is therefore necessary to use printing ink that, when printed, is particularly effective on a wide selection of plastic materials by way of an inkjet printer as described in EP 0 419 442 B1, these inks possessing a defined curing, fixing, or hardening phase that becomes effective upon exposure of the ink to UV radiation. When UV-curable inks of this kind, as described in DE 69909 3322 T2, are used, it is necessary, in order to avoid printing defects, to equip the inkjet printer with an ink removal apparatus, as described in DE 10 2004 058 084 A1, that removes the residual ink from an inkjet printing head. The ink removal apparatuses known from the existing art encompass only wiping elements that skim over the nozzles of the inkjet printing head. An optimized ink removal configuration, which also enables cleaning of the nozzle device by means of a pump, is therefore required.